Transmission regulation



June 29 1926. 1,590,362.

Y R. D. GIBSON TRANSMI SS ION REGULATION m Filed April 24, 1924 Sfahon E Srafion W Patented June 29, 1926.

UNITED STATES 1,590,362 PATENT OFFICE.

ROBERT D. GIBON, 0F RAHWA'Y, NEW JERSEY, ASSIGNOR T0 WESTERN ELECTRIC COM- IPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW. YORK.

TRANSMISSION REGULATION.

Application filed April 24, 1924. Serial No. 708,604.

This invention relates to transmission regulation and is particularly adapted for use in multiplex carrier current signaling systems.

An object of the invention is to stabilize the transmission in carrier signaling systems.

A related object of the invention is to utilize a signal channel of a multiplex carrier signaling system for the transmission of a control or pilot current without interfering with the normal transmission of signals thereover. y

Another object of the invention is to provide a flexible gain control system which may be used in connection wth any channel of a multiplex signaling system.

The invention is particularly applicable to signaling systems employing fairly high frequencies in which, as is well known, the transmission characteristics of the transmission medium are subject to variations which render it difiicultto maintain uniform signalirig conditions.

Changes in the attenuation of the transvmission medium of a system of the above type produce corresponding variations in the over-all transmission equivalent of the system, and result in large fluctuations in the volume of a telephone or other message being transmitted.

In order to overcome the effects of transmission variations in such systems, it is common practice to transmit a control or pilot current between the terminals of the system to indicate the condition of the transmission medium over which the telephone, telegraph, or other signals are transmitted. This pilot current may be utilized at the receiving terminal to operate an indicating device or an automatic compensator, or, if desired, both indicating and compensating devices. Heretofore, it has been thought necessary to set aside one signal channel for the exclusive transmission of the pilot current, thus reducing the frequency range available for the transmission of signals.

According to a feature of the invention, a control or pilot current is transmitted along with the voice or other signal modulation on one channel of a multiplex signaling system without in any way interfering with the normal transmission of signals over this channel.

More specifically, one channel of the system is utilized for the transmission of a pilot current of low frequency in addition to 1ts normal use as a signal channel. The pilot current which may be of about 200 cycles frequency, is impressed upon the transmission medium at the transmitting terminal along with the signal currents. At the receiving terminal, the pilot current is selected and is utilized to operate an indicating device to indicate changes in the transmission characteristics of the transmission medium.

The invention will be described as applied to a multiplex carrier current telephone system, although it will be understood that it may also be applied to other signaling systerns.

The drawing shows a diagrammatic il-" lustration of a multiplex carrier telephone signaling system embodying the invention.'

The system shown in the drawing is represented as comprising two terminal stations W and E, respectively, interconnected by the multiplex line ML. If desired, one or-more repeater stations (not shown) may be connected to the line ML intermediate the terminal stations.

The terminal carrier current apparatus at station W comprises a plurality of transmitting. channels TG,, TC etc., connected through a common transmitting circuit TL to the main line ML, and a plurality of receiving channels RC RC etc. connected through a common receiving circuit RL, to the main line ML. At station E, transmitting channels TC T6 etc. are connected to the main line through a common transmit-ting circuit TL and receiving channels RC RC etc. arecOnnected to the main line through a common receiving circuit RL Carrier currents are utilized for transmission over the line ML and. are grouped as to their frequencies, the lower frequencies, as a group, being used for transmission from station TV to station E, and the higher frequencies, as a group, being used for transmission from station E to station W.

Low pass grouping filters LP and LP at the respective terminal stations are each paired with a high pass filter HP and HP,. These filters serve to separate the directional groups of carrier waves to the respective terminal transmitting and receiving circuits. These filters and each of the other filters shown throughout the system may be constructed and designed in accordance with the principles laid down in the United States Patent to Campbell, No. 1,227,113, issued May 22, 1917.

F or the purpose of the present description, a low pass filter may be taken to be any filter which transmits freely currents of all frequencies between zerov and a preassigned limiting frequency, and suppresses the transmission of currents of all frequencies higher than ,said limiting frequency. Conversely, a high pass filter may be taken to be any filter which freely transmits currents of all frequencieshigher than a preassigned limiting frequency and suppresses currents of all frequencies lower than said limiting frequency. Specific types of both of these kinds of filters are shown, for example, in Fig. 11 of the above mentioned Campbell patent.

Low frequency lines L and L which may be telephone lines or other types of signaling lines are associated with channels TC RC and T0,, RG respectively at station W for simultaneous and independent communication over the line ML with corresponding similar lines L and L which are associated with the channels TCQ, RC and TC, R0, at station E.

Each low frequency line is provided with a balancing artificial line or network N and with a difierential repeating coil H, commonly knownas a hybrid coil, for enabling independent transmission in the two directlons between the line and the high frequency terminal apparatus.

The transmitting channel TC at stationv W includes a modulator M and a band filter TBF while transmitting channel TC includesa modulator M, and a band filter TBF The receiving channel R0, at this station includes a demodulator D and a band filter RBF while receiving channel RC includes a demodulator D, and a band filter RBF At station E, the transmitting channels TC and TC include the respective modulators M and M and band filters TBF and TBF and the receiving channels RC and RC, include the respective demodulators D and D and band filters RBF and RBF,.

The modulators and demodulators associated with the various transmitting and re" ceiving channels throughout the system may be of any well known type, such as the balanced modulator and demodulator disclosed in the U. S. patent to Carson, No. 1,343,306, issued June 15, 1920.

The carrier system described above is of thetype in which the carrier wave of each channel is suppressed from transmission when no signals are being sent,and when signals are being transmitted modulation component of the modulated wave are sent.

In order to reproduce the signal from modulated waves ofthis character, it is necessary-that the carrier wave supplied to the demodulator at the receiver be of substan-' waves cof the same frequency, although so far as the present invention is concerned, any type of frequency control may be used.

Carrier currents of the frequency assigned to channel ,TC are su plied from a source TO, to the modulator ll, wherein they are modulated by voice frequency currents or other signaling currents from the low frequency line L Carrier sources R0,, R0, and T0, are associated with the demodulators D and D and modulator M at station W, while carrier current sources T0 TO, and R0,. ROJ are associated with'the modulators M M and demodulators D 1),, respectively, at station E.

As noted above, the carrier current sources R0 R0 R0,, and R0,, are necessaryto supply to the incoming side bands in each receiving channel, the carrier current which was suppressed at the transmitting end of the channel.

The various band filters associated with the transmitting and receiving channels at stations W and E ma be of the band filter type disclosed in the ampbell patent, supra. These filters are so designed that they will transmit bands of frequencies in the neighborhood of the carrier frequency assigned to their particular channel, the bands being of sufiicient width to accommodate the fre-- quency variation due to the modulation of the basic carrier frequency by the signaling frequencies. The bands transmitted by the filters associated with the various transmitting and receiving channels should be sufliciently remote from the bands transmitted bythe filters associated with the other chan nels to allow for frequency separation be tween the different channels.

At station W, an amplifier TA is in cluded in circuit with the common transmitting circuit TL and low pass grouping filter LP,, and a receiving amplifier RA is included in circuit with the common receiving 'circuit- BL and the high pass grouping filter HP At station E, transmitting amplifier ,TA, is included in circuit with the common transmitting circuit TL and grouping filter HP,, and receiving amplifier RA, is included in circuit with the common receiving circuit RL and grouping filter LP a In accordance with the invention, a current of low frequency is transmitted as a pilot current along with the voice currents on any signaling channel. of the system. This pilot current which-may be of about 200 cycles frequency, may be impressed upon any of the transmitting channels, such as TC, at station WV, is combined in the modulator M with the current of carrier frequency supplied by the source TO, and is transmitted over the line to station E. At station E, the speech-and-pilot modulated current is selected by the band filter in a previously assigned receiving channel,

such as channel RC is demodulated, and

the low frequency pilot current is then selected by a low pass filter and utilized to operate an indicating or recording device.

At station W, the pilot frequency current is supplied from a pilot channel PC connected to the low frequency line L The pilot channel PC includes a source of pilot current 10, a low pass filter 11 designed to transmit currents from zero to the pilot frequency, and an adjustable current regulator 12. A thermocouple'14 may be included in the pilot channel PO, to supply a current proportional to the output of the pilot source .10 to an ammeter 1 3 which serves to give a continuous indication of the pilot current. This enables the pilot current supplied to the modulator to be kept constant, as by varying the tap on'resistance 12, either automatically or manually, to keep the reading of the ammeter 13 constant.

At station E, a pilot channel PC is connected to the low frequency line L The pilot channel PC includes a low pass filter 15 identical to the low pass filter 11, an adjustable current regulator 16, amplifier 17, thermocouple 18, and an associated indicating device 19. The indicating device 19 may be any suitable type of meter such as a galvanometer, and it is adapted to be operated by the direct current E. M. F. which is produced between the terminals of the thermocouple 18. This E. M. F. is a function of the amplitude of the alternating current received by the pilot channel PC and any change in this alternating current produces a change in the direct current output of the thermocouple 18.

A high pass filter 20' is included in each of the low frequency lines L and L These filters are so designed that they transmit currents within the voice range but suppress the transmission of current of the unmodulated pilot frequency to the low fre-' quency telephone apparatus associated with these bw frequency lines. These filters also mitted through the low pass filter 11 in pilot channel PC to the low frequency line L and thence to the transmitting channel T0,. The pilot current, along with speech currents from line L,, is combined, in the modulator M with the carrier current supplied by the source T0,, and one side band produced by this modulation, for example the lower side band, is transmitted through the filter TBF amplifier TA,, and low pass grouping filter LP to the line llfL.

At station E, the modulated wave is transmitted by the low pass grouping filter LP and amplifier RA to the common receiving circuit RL and is selected by the band filter RBF in the receiving channel RC The receiving wave is then passed to the demodulator D where it is combined with a current of carrier frequency supplied by the source R0 to reproduce speech currents and a low frequency current of 200 cycles, the original pilot frequency. The detected speech currents are transmitted through filter '20 to the line L,. The pilot current is selected by the low pass filter 15, is transmitted through the amplifier 17 and is impressed upon thermocouple 18 in circuit with the meter 19.

If a change in line equivalent occurs anywhere in the system, the incoming pilot current changes and produces a change in the potential derived from the thermocouple 18 which produces a corresponding deflection of the index of the meter 19.

If desired, an indicating device having a l recording chart and associated mechanisms for making a permanent record may be employed instead of the meter 19. One type of apparatus for accomplishing this result is shown and described in the United States patent to Brewer, No. 1,356,804, issued October 26, 1920.

The meter 19 may also be employed to automatically control adjusting elements in the transmission circuit to compensate for changes of line equivalent.

The system described above provides means for indicating the transmission equivalent of the signaling system as a unit from voice frequency line to voice frequency line including all the carrier terminal apparatus.

It will be seen that by means of this invention, any channel of a carrier signaling system may be used to transmit a pilot current. Furthermore, since the pilot channel apparatus operates at a low frequency, diflis I appended claims.

What is claimed is:

1. In a transmission system, the method which comprises transmitting signal and control waves over a variable transmission medium as modulations of the same carrier wave, and utilizing the control Wave so transmitted to indicate changes in the transmission characteristics of said'medium.

2. In a transmission system,-tlie metl1od Which comprises modulating a Wave derived from a source at a sending station by speech Waves, modulating a Wave from said source by a control Wave of a frequency below the essential speech frequencies, transmitting products of such modulation over a variable transmission medium, and utilizing the control wave so transmitted to indicate the transmission condition of said medium.

3. In a transmission system, the method which comprises modulating a wave derived from a source at a sending station by a Sig nal wave, transmitting a product of the wave so modulated over one channel of a variable transmission medium, modulating 'a wave derived from said source by a control wave,

transmitting the wave so modulated by the control Wave over said channel; and utilizing the control wave so transmitted to indicate changes in the transmission equivalent of said medium.

4. In a transmission system, the method which comprises modulating a wave derived from a source at a sending station by a signal wave, transmitting a product of the wave so modulated over one channel of a variable transmission medium, modulating a wave derived from a source at said sending station by a control Wave, transmitting a wave derived from such modulation over said channel, and utilizing the control wave so transmitted to indicate changes in the transmission equivalent of said medium.

5. In a transmission system the method which comprises transmitting a signal wave over a variable transmission medium, superimposing a control Wave upon said signal Wave, and utilizing the control wave so transmitted to indicate changes in the transmission equivalent of said medium.

6. Ina system for indicating variations in the attenuation of a transmission medium, a transmitting station including a source of carrier waves, means for modulating Waves from said source in accordance with signalwaves, means for also modulating Waves from said source in accordance with a tors, means for supplying low frequency signal waves to said modulators, means for supplying a low frequency control Wave of constant amplitude to one of said modulators, detector in the high frequency channels I at said receiving station for combimng the, 35

received modulated Waves with locally generated waves to reproduce the low frequency modulating Waves, means associated with one of said receiving channels toselect the reproduced control wave, and an indicating device for indicating variations in amplig "tude of the received control Wave.

8. In a system for indicating variations in transmission equivalent, a transmitting station and a receiving station, a variable transmission medium connecting said stations, a scource of high frequency Waves and a source of low frequency signal Waves at said transmitting station, means for modulating a wave derived from said high frequency source by a wave derived from said low frequency signal source, means for impressing a product of the wave so modulated upon said transmission medium, a source of low frequency control waves at said transmitting station, means for modulating a Wave derived from said high frequency source in accordance with a Wave of constant amplitude derived from said low frequenc control source, means for-suppressing t e un- 9. In a multiplex carrier telephone system comprising at one station modulating circuits for producin and transmitting a single side band ofeac of a plurality of speech modulated vwaves of respective frequency ranges and at another station receiving circuits for deriving from each received side band a different speech wave, means for indicating changes in the transmission characteristics of said system including at said one station a source of low frequency currents, 1 0

signal and 1 5 a means to supply a wave of substantially constant amplitude from said'source to one of said modulating circuits whereby the resulting transmitted side band from that modulating circuit contains a component corresponding to said low frequency current,

means at said other station to derive from the particular side band so transmitted a current of said low frequency, and means to measure the amplitude of said derived low frequency current from instant to instant to indlcate variations in the transmission characteristics.

In witness whereof, I hereunto subscribe my name this 21 day of AprilA. D., 1924. ROBERT D. GIBSON. 

